The present invention relates to internal-combustion engines of the type comprising:
at least one induction valve and at least one exhaust valve for each cylinder, each valve being provided with respective elastic means that bring back the valve into the closed position to control communication between the respective induction and exhaust ducts and the combustion chamber;
a camshaft for operating the induction and exhaust valves of the cylinders of the engine by means of respective tappets;
in which at least one of said tappets controls the respective induction or exhaust valve against the action of said elastic return means via the interposition of hydraulic means including a hydraulic chamber containing fluid under pressure;
said hydraulic chamber containing fluid under pressure being connectable, via a solenoid valve, to an outlet channel for decoupling the valve from the respective tappet and causing fast closing of the valve under the action of respective elastic return means;
said hydraulic means further comprising a piston associated to the stem of the valve and slidably mounted in a guide bushing, said piston being set facing a variable-volume chamber defined by the piston inside the guide bushing, said variable-volume chamber being in communication with the hydraulic chamber containing fluid under pressure by means of an end aperture of said guide bushing, said piston having an end appendage designed to be inserted into said end aperture during the final stretch of the closing stroke of the valve in order to restrict the communication port between said variable-volume chamber and said hydraulic chamber containing fluid under pressure, so as to slow down the stroke of the valve in the proximity of its closing.
An engine of the type referred to above is, for example, described and illustrated in the European patent applications Nos. EP-A-0 803 642 and EP-A-1 091 097 filed by the present applicant.
Studies and tests carried out by the present applicant have shown that some problems may arise during operation, and in particular noise on account of the play that may arise between the various parts both as a result of the constructional tolerances and on account of wear. In particular, it has emerged that in the course of operation of the engine, the ring which functions as a seat for the engine valve and which is received into a cavity of the engine cylinder head may undergo displacements of one or two tenths of a millimeter following upon the continuous impact of the head of the valve against the ring. So far this problem has been solved by using pads for compensating the play.
The purpose of the present invention is to overcome the above-mentioned problems.
With a view to achieving this purpose, the subject of the invention is an engine having all the characteristics referred to at the beginning of the present description and characterized moreover in that set between the stem of the valve and the aforesaid piston for actuating the valve is an auxiliary hydraulic tappet.
In the practical implementation, the aforesaid auxiliary hydraulic tappet comprises an auxiliary piston which is slidably mounted in the body of the actuating piston, has one end set inside the actuating piston and set facing a chamber within the actuating piston, the said chamber being in communication with the chamber containing fluid under pressure of the system for controlling the valves, and one end set outside the actuating piston, which is in contact with the end of the valve stem, elastic means being provided for bringing back said auxiliary piston into an end-of-stroke position in the direction of the valve stem.
Inside the aforesaid chamber made within the actuating piston, a non-return valve is set which enables passage of fluid under pressure coming from the hydraulic pressure chamber inside the chamber of the auxiliary hydraulic tappet.
As emerges clearly from the foregoing description, in the engine according to the invention, the stem of the valve is not rigidly connected to the actuating piston, given that set between them is the aforesaid auxiliary hydraulic tappet, which is thus able to recover all the possible play that may arise as a result of the fabrication tolerances or wear of the parts.
The arrangement according to the invention may be adopted both for the induction valves and for the exhaust valves, but is particularly useful in the case of the exhaust valves, in that the problems referred to above tend to occur more easily for this type of valve.